PCI-E Cables

I don't think I understand what you mean... the Mac Pro already has two power connectors... but you say PCIe cables?

 

Here is a guide

http://forums.macrumors.com/showthread.php?t=418279


And btw, PCI connectors are soldered directly on the motherboard, there are no such things as PCI-cables. Although, you will require extra power cables for the 8800GTX. All of that is covered in my guide.

And remember that you will not benefit from putting 2 graphic cards in the Mac Pro, as you can only run 1 card in normal speed (16x) or 2 cards at half speed (8x).